Thermal head
BACKGROUND OF THE INVENTION 
     The present invention relates to a thermal head which is suitably used for a thermal printer. 
     A typical example of a portion of the thermal head according to the prior art is illustrated in FIGS. 1 and 2, in which an aluminum oxide substrate 1 has a glass glaze layer 2 coated over its upper face. Coated over the upper face of the glass glaze layer 2 is a resistance heating element 3, over which are formed electrodes 4a, 4b at a predetermined distance, thus a portion of the heating element 3, which is located below the gap between the electrodes 4a, 4b serving as a heating portion 3a. One of the electrodes 4a, 4b is grounded and the other is connected to an output terminal of a power supply control unit (not shown), which supply electric current to the heating portion 3a. The reference numeral 5 indicates an oxidation-resistant film to cover both the electrodes 4a, 4b and the heating portion 3a. The oxidation-resistant film 5 is coated with a wear- or abrasion resistant film 6. The oxidation-resistant film 5 and the abrasion-resistant film 6 constitute a protection film. FIG. 2 illustrates a plan view of the heating portion 3a when the protection layer is removed. When current is supplied to each of heating portions 3a of the thermal head, it is uniformly heated, so that the shape of a dot of a picture is reproduced as shown by the broken line in FIG. 2, for example, in a thermal paper. 
     In the thermal printer, it is preferable to change the size of dots, reproduced on a printing paper, for expressing a picture in gradation. However, the thermal head above described cannot make such dot printing, and the printing thereof is restricted by the shape of each heating portion 3a. 
     Accordingly, it is an object of the present invention to provide a thermal head which is capable of changing the size of dots of a picture in printing. 
     SUMMARY OF THE INVENTION 
     With this and other objects in view, the present invention provides a thermal head for use in thermal printing including: a substrate having an upper face; an electrically insulating layer, coated over the upper face of the substrate; heating means, coated over the insulating layer, for providing heat for printing a dot of a picture; a protection layer, coated over the heating means; and dot area control means, formed above the heating means to receive sufficient heat for the printing, for transferring the heat from the heating means upwards for the printing of the dot and for controlling an area of the dot. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, in which like reference characters designate corresponding parts throughout several views and descriptions thereof are omitted after once given: 
     FIG. 1 is an enlarged partial vertical cross-section of the thermal head according to the prior art; 
     FIG. 2 is an enlarged plan view of the thermal head in FIG. 1, some components thereof being removed for illustrating the heating portion; 
     FIG. 3 is an enlarged partial plan view of a thermal head of the present invention, some components thereof being taken away for showing a heating portion thereof; and 
     FIG. 4 is a vertical cross-section of the thermal head taken along the line A--A in FIG. 3, no part of the thermal head being removed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 3 and 4, the reference numeral 7 designates a resistance heating element layer of a conventional material, coated over a glass glaze layer 2. The resistance heating element layer 7 has electrodes 4a and 4b coated over it at predetermined interval. A bridge like portion 7a, which is located just below the gap between the electrodes 4a and 4b, constitutes a heating portion. The heating portion 7a is smaller in width than the other portion of the resistance heating element 7. The heating portion 7a and the electrodes 4a, 4b are coated with an oxidation resistance film 5, made of a conventional material such as SiO 2 , on which thermal transfer aluminum members C1, C2a, C3a, C4a, C2b, C3b, C4b are formed at the same level with equal thickness, about 1-2 μm in this embodiment. The thermal transfer member C1 has a rectangular shape, equal in width, to the heating portion 7a and is located just above the center of the heating portion 7a. A pair of thermal transfer members C2a and C2b have a channel-shape or generally C-shape in plan view. The thermal transfer members C3a C3b, C4a and C4b have configurations similar to the thermal transfer members C2a and C2b. The thermal transfer members C2a and C2b are the smallest among the thermal transfer members C2a-C4a, C2b-C4b and are symmetrically arranged about the center line of the thermal transfer member C1 or the A--A line in an equi-spaced manner from the thermal transfer member C1 so that they surround the latter. The thermal transfer members C4a and C4b are the largest of the thermal transfer members. The thermal transfer member pair C3a, C3b is symmetrically arranged with equal spacing from respective thermal transfer members C2a and C2b in the same manner as the latter so that they surround the thermal transfer members C2a and C2b. Also, the thermal transfer members C4a and C4b are symmetrically disposed with an equal interval from respective thermal transfer members C3a and C3b so that they surround the latter. The reference numeral 6a indicates a conventional abrasion resistant film made of a conventional material such as Ti 2  O 5 , and coated over both the thermal transfer members C1, C2a-C4a, C2b-C4b and the oxidation resistance film 5. The upper surface of the abrasion resistance film 6a has V-shaped grooves 8 formed in it so that bottoms of the grooves 8 pass just above the center of the space of adjacent two thermal transfer members. Thus, projections 9, which have shapes similar to corresponding thermal transfer members, are defined by grooves 8. 
     For a thermal head for 200 dpi printing, the width of the thermal transfer members C2a-C4a, C2b-C4b may be about 5 μm, and the gap between adjacent two thermal transfer members may be about 3 μm. 
     When suppled with current, the heating portion 7a of the thermal head is uniformly heated to elevate temperatures of the thermal transfer members C1, C2a-C4a and C2b-C4b. The temperatures of these thermal transfer members depend on both the distance from the heating portion 7a and the area thereof. Thus, the innermost thermal transfer member C1 becomes the hottest, and the thermal transfer members drop in temperature from the innermost thermal transfer member C1 toward the outermost thermal transfer members C4a and C4b. Control of both current supply time and magnitude of current makes it possible to heat the thermal transfer member C1 or both the thermal member C1 and other specific thermal transfer members to temperatures sufficient for melting an ink of a ribbon in a thermal transfer printer or to temperatures necessary to turn a portion of a thermal paper to be a black dot. The size of each dot reproduced in a printing paper is, thus, controlled and hence gradation of the picture tone is achieved by changing the size of dots. 
     In place of the channel-shaped thermal transfer members C2a, C2b, C3a, C3b, C4a and C4b, each pair of the transfer members may be formed integrally into an annular shape. 
     The thermal transfer members C1, C2a-C4a and C2b-C4b may be made of other materials, having good thermal conductivity, such as copper, gold and silver. 
     The thermal transfer members C1, C2a-C4a and C2b-C4b may be arranged outside the protection film, for example, on the upper surface of the abrasion resistant film 6a.